As children are taught at school, trees and other plants absorb CO2 from the air as they grow. Using energy from the sun, they turn the carbon captured from the CO2 molecules into building blocks for their trunks, branches and foliage. This is all part of the carbon cycle.

A mature forest doesn't necessarily absorb much more CO2 that it releases, however, because when each tree dies and either rots down or is burned, much of its stored carbon is released once again. In other words, in the context of climate change, the most important thing about mature forests is not that they reduce the amount of CO2 in the air but that they are huge reservoirs of stored carbon. If such a forest is burned or cleared then much of that carbon is released back into the atmosphere, adding to atmospheric CO2 levels.

Of course, the same process also works in reverse. If trees are planted where previously there weren't any, they will on soak up CO2 as they grow, reducing the amount of greenhouse gas in the atmosphere. It is thought that trees, plants and other land-based "carbon sinks" currently soak up more than a quarter of all the CO2 that humans add to the air each year – though that figure could change as the planet warms.

Unsurprisingly, the relationship between trees and local and global temperature is more complicated than the simple question of the greenhouse gases they absorb and emit. Forests have a major impact on local weather systems and can also affect the amount of sunlight absorbed by the planet: a new area of trees in a snowy region may create more warming than cooling overall by darkening the land surface and reducing the amount of sunlight reflected back to space.